ESTIMATING POLYACRYLAMIDE CONCENTRATION IN IRRIGATION WATER

One practice used to control irrigation-induced erosion amends irrigat ion furrow inflows with water-soluble, anionic polyacrylamide (PAM) at low concentrations (0.25-10 mg L(-1)). Researchers wish to determine the fate of PAM, once added to furrow water streams. We developed and tested an instrumented flocculation test for quantifying PAM concentra tion in irrigation water. A kaolinite mineral standard is mixed with a PAM-amended water sample, agitated, then placed in a spectrophotomete r. The PAM concentration in the suspension was correlated with settlin g-related transmittance changes. One highly correlated (r = 0.91-0.98) parameter, the time needed to initiate suspension clearing (clarity-s hift inflection, CSI), was used as the procedure endpoint. The procedu re was sensitive to variations in the amount of kaolinite added, and s ample volume, water salinity, and original sediment content. A 10% cha nge in these factors altered measured CSIs by 10 to 50%. The sediment affected CSI by increasing the sample's dissolved organic C concentrat ion. The procedure detected as little as 0.1 mg L(-1) PAM dissolved in irrigation water; in samples containing > 4 mt settled sediment per l iter, the PAM detection limit was approximately 0.25 mg L(-1). Precisi on ranged from +/- 0.06 to 0.11 mg L(-1) for 0 to 2.5 mg L(-1) PAM and +/- 0.39 to 0.86 mg L(-1) for 2.5 to 10.0 mg L(-1) PAM. The PAM conce ntration in runoff from irrigated furrows equaled that of the inflow s tream after 3 h continuous treatment at 10 mg L(-1). The CSI test prov ides a simple and accurate method of determining polyacrylamide in sur face waters.